1. Field of the Invention
This invention generally relates to spread spectrum code division multiple access (CDMA) communication systems. More particularly, the present invention relates to a system and method for adaptively limiting forward and reverse link transmission power within CDMA communication systems.
2. Description of the Prior Art
Wireless communication systems using spread spectrum modulation techniques represent the state of the art in digital communications and are increasing in popularity. In code division multiple access (CDMA) systems, data is transmitted using a wide bandwidth (spread spectrum) by modulating the data with a pseudo random chip code sequence. The advantage gained is that CDMA systems are more resistant to signal distortion and interfering frequencies in the transmission channel than communication systems using other multiple access techniques such as time division multiple access (TDMA) or frequency division multiple access (FDMA).
One indicator used to measure the performance of a communication system is the signal-to-noise ratio (SNR). At the receiver, the magnitude of the desired received signal is compared to the magnitude of the received noise. The data within a transmitted signal received with a high SNR is readily recovered at the receiver. A low SNR leads to loss of data.
A prior art CDMA communication system is shown in FIG. 1. The communication system has a plurality of base stations 201, 202 . . . 20N connected together through a local Public Switched Telephone Network (PSTN) exchange. Each base station 201, 202 . . . 20N communicates using spread spectrum CDMA with mobile and fixed subscriber units 221, 222 . . . 22N within its cellular area.
Shown in FIG. 2 is a simplified CDMA transmitter 24 and receiver 26. A data signal having a given bandwidth is mixed with a spreading code generated by a pseudo random chip code sequence generator producing a digital spread spectrum signal for transmission. Upon reception, the data is reproduced after correlation with the same pseudo random chip code sequence used to transmit the data. By using different pseudo random chip code sequences, many data signals or subchannels can share the same channel bandwidth. In particular, a base station 201 can communicate with a group of subscriber units 221, 222 . . . 22N using the same bandwidth. Forward link communications are from the base station 201 to the subscriber unit 221, 222 . . . 22N, and reverse link communications are from the subscriber unit 221, 222 . . . 22N to the base station 201.
For timing synchronization with a receiver 26, an unmodulated pilot signal is used. The pilot signal allows respective receivers 26 to synchronize with a given transmitter 24, allowing despreading of a traffic signal at the receiver 26. In a typical CDMA system, each base station 201, 202 . . . 20N sends a unique global pilot signal received by all subscriber units 221, 222 . . . 22N within communicating range to synchronize forward link transmissions. Conversely, in some CDMA systems for example in the B-CDMA™ air interface each subscriber unit 221, 222 . . . 22N transmits a unique assigned pilot signal to synchronize reverse link transmissions.
FIG. 3 is an example of a prior art transmitter 24. Data signals 281, 282 . . . 28N including traffic, pilot and maintenance signals are spread using respective mixers 301, 302 . . . 30N with unique chip code sequences 321, 322 . . . 32N, respectively. Each mixers' output is coupled to a combiner 34 which adds the individual mixed signals as a combined signal 44. The combined signal 44 is modulated up to radio frequency (RF) by a mixer 36 mixing the combined signal 44 with an RF carrier, shown in FIG. 3 as COS ωt. The modulated signal is amplified to a predetermined transmission power level (TLP) by an amplifier 38 and radiated by an antenna 40.
Most CDMA systems use some form of adaptive power control. In a CDMA system, many signals share the same bandwidth. When a subscriber unit 221, 222 . . . 22N or base station 201, 202 . . . 20N is receiving a specific signal, all the other signals within the same bandwidth are noiselike in relation to the specific signal. Increasing the power level of one signal degrades all other signals within the same bandwidth. However, reducing TLP too far results in undesirable SNRs at the receivers 26. To maintain a desired SNR at the minimum transmission power level, adaptive power control is used.
Typically, a transmitter 24 will send a signal to a particular receiver 26. Upon reception, the SNR is determined. The determined SNR is compared to a desired SNR. Based on the comparison, a signal is sent in the reverse link to the transmitter 24, either increasing or decreasing transmit power. This is known as forward channel power control. Conversely, power control from the subscriber unit 22, to the base station 20, is known as reverse channel power control.
Amplifiers 641, 642 . . . 64n are used for adaptive power control in FIG. 3. The amplifiers 641, 642 . . . 64n are coupled to the inputs of the combiner 34 to individually control each signal's power level.
FIG. 4a, 4b, 4c and 4d show a simplified illustration of three spread spectrum signals 421, 422, 423 and a resultant combined signal 44. Although each signal 421, 422, 423 is spread with a different pseudo random chip code sequence, each signal 421, 422, 423 is synchronous at the chipping rate. When the individual chips within the sequences are summed, the combined signal may have extreme transients 46, 48 where the chip energies combine or low transients 47 where they subtract.
High transient peaks are undesirable. For every 3 dB peak increase, twice the base amplification power in Watts is required. Not only does the transient burden the amplifier, but the power sourcing the amplifier must have a capacity greater than the maximum transient that may be expected. This is particularly undesirable in hand-held battery operated devices. Additionally, to design for higher power levels resulting from high transients, more complex amplifier circuitry is required or compromises between amplifier gain, battery life and communication time result. High valued transients force the amplifier 38 into the nonlinear region of its dynamic range resulting in increased out-of-band emissions and reduced amplifier efficiency. Accordingly, there exists a need for an adaptive RF transmitter system that addresses the problems associated with the prior art.